well personally if you have the money to spend, get the 5850. there is a law in computer parts buying that i always tell my clients and customers.
the longer you wait to buy parts, the cheaper and better the parts are. and always buy the best parts you can afford at that time.
basically if you wait a bit. the price will drop more on the 5850 (especially when the 5950 hits. wowza what a card) if you give it 3 months i bet it will be $50 cheaper. also, if you can afford it, spend it. the benchmarks on those two cards are close but night and day for some specs.
now that being said. the 5770 card is a fine card. excellent for the price (bear in mind almost all cards are close to $500-600 upon release) and drop drastically over time. i like to call it the "early adopter tax" if you really want to go nuts. buy two 5770's for less than the price of 1 5850 and crossfire them. the stats on two 5770's in crossfire blow the 5850 away!!!